1. Field of the Invention
The present invention relates to a method of manufacturing an electronic circuit board on which LSIs, components, etc. to be used in an electronic circuit board are mounted, and particularly to a method of manufacturing an electronic circuit board on which these LSIs, components, etc., are mounted by means of a lead-free Bi-system solder. The solder alloy can be applied in bonding electronic components such as LSIs to a circuit board made of an organic material and is an alternative to conventional eutectic Pb--Sn alloy solders used in the soldering at a temperature of from 220.degree. C. to 230.degree. C.
2. Description of the Related Art
There have been problems of pollution of the global environment and adverse affects on living things by lead (Pb) in a eutectic Sn-Pb solder. It is thought that the pollution of the global environment occurs when lead is dissolved by rain, etc. from the dumped lead-containing electrical appliances exposed to sunlight and rain. The dissolution of lead tends to be accelerated by the recent acid rain. In order to reduce environmental pollution, therefore, it is necessary to use a lead-free solder alloy of low toxicity containing no lead as an alternative for the eutectic Sn--Pb alloy solder which is popular. This demand can be solved by using lead-free solder alloys which contain Bi, etc., such as an Sn--Ag--Bi alloy and an Sn--Ag--Bi--Cu alloy.
However, the lead-free solders containing Bi, etc. have had a problem of lacking reliability because of exfoliation at joint portions. Furthermore, the lead-free solders have had the problem that deterioration of strength and exfoliation occur due to heating of surrounding bonding portions (which do not require repairs) during repairing.
More specifically, a phenomenon called "lift-off" is known when a lead-free Bi-system solder is used in soldering a printed circuit board. The phenomenon is of exfoliation between a land of the board and the solder that occurs after soldering. It has been known that the lift-off occurs due to segregation of Bi in a Sn--Bi alloy solder (Collection of Lectures and Papers on Circuit Mounting at Grand Scientific Lecture Meeting, p. 67, Mar. 24, 1998). However, the mechanism of the lift-off is complex and has been uncertain. Further, it is shown in a News Paper of the Nikkan Kogyo Shinbun issued on of Apr. 27, 1998 that the lift-off can be prevented by rapid cooling using water or a steam spray.
Thus, a problem to be solved by the present invention is to prevent deterioration of strength and the lift-off phenomenon at bonding portions of lead-free Sn--Ag--Bi alloy solders having a low melting point, etc. and to prevent occurrence of deterioration of strength and the lift-off of surrounding bonding portions (which do not require repairs) during repairing. Namely, when soldering is performed in an ordinary flow process by means of a lead-free Bi-system solder, the lift-off occurs between the solder and a land. If the lift-off can be prevented, it is possible to perform soldering at a temperature comparable to the soldering temperature for the conventional eutectic Sn-Pb alloy solder. A problem of damage to the solder and components and an adverse effect due to a cooling liquid (especially, corrosion of contact portions) arises simply in the case of rapid cooling.